In a wireless cellular communications system, multiuser multiple-input multiple-output (MU-MIMO) is a promising technique to significantly increase the cell capacity. In MU-MIMO, the signals intended to different users are simultaneously transmitted with orthogonal (or quasi-orthogonal) precoders. On top of that, the concept of a joint optimization of multiuser operation from both transmitter and receiver's perspective has the potential to further improve multiuser system capacity even if the transmission/precoding is non-orthogonal which could come from, for example but not limited to, the simultaneous transmission of a large number of non-orthogonal beams/layers with the possibility of more than one layer of data transmission in a beam. Such non-orthogonal transmission could allow multiple users to share the same resource elements without spatial separation, and allow improving the multiuser system capacity for networks with a small number of transmit antennas (i.e. 2 or 4, or even 1), where MU-MIMO based on spatial multiplexing is typically limited by wide beamwidth. An example of such joint Tx/Rx optimization associated with adaptive Tx power allocation and codeword level interference cancellation (CWIC) receiver is recently a remarkable technical trend, including non-orthogonal multiple access (NOMA) and other schemes based on downlink multiuser superposition transmission (MUST).
When increasing of antenna number with multi-user transmission, capacity is expected to grow. However, limited feedback information results in non-ideal beamforming and MU paring, MU interference limits the capacity grow. Interference cancellation (IC) may be the tool to improve capacity region. For MU-MIMO, both cell average spectral efficiency and cell edge spectral efficiency improve when codeword level interference cancellation (CWIC) is used.
Interference problems exist for massive MU-MIMO under different deployment scenarios. For non-ultra-dense scenario, MU transmission is via different beams. Interference comes from sidelobe, reflection, diffraction, or non-ideal beamforming. There is certain interference and IC is still helpful. For ultra-dense scenario, MU transmission is via the same beam (i.e., multi-user superposition transmission (MUST)). It is difficult to separate signals in spatial domain due to crowded user. Wider beamwidth by <6 GHz massive MIMO antenna results in worse interference. Interference cancellation capability can significantly improve system capacity. Other interference problems exist in cellular networks. For example, inter-cell interferences come from neighbor cells for cell edge users, and DL-to-UL and UL-to-DL interferences result from dynamic time division duplex (TDD) configuration.
A new air interface that is interference cancellation friendly is desired.